Constructor parameter that have the forms of a forwarding reference (T&&) inside a constructor are not forwarding reference if T is a parameter of the class template. But gcc assumes it is a forwarding reference if the constructor is defined outside the class: template<typename T> struct B1 { B1(T&& x){} }; template<typename T> struct B2 { B2(T&& x) ; }; template<typename T> B2<T>::B2(T&& x){} int i; B1 a{i};//Expect: compilation error, class template argument deduction failed B2 b{i};//Unexpected: no compilation error.
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>: https://gcc.gnu.org/g:bebd8e9da838c51a7f911985083d5a2b2498a23a commit r12-2309-gbebd8e9da838c51a7f911985083d5a2b2498a23a Author: Patrick Palka <ppalka@redhat.com> Date: Wed Jul 14 15:37:30 2021 -0400 c++: CTAD and forwarding references [PR88252] Here during CTAD we're incorrectly treating T&& as a forwarding reference even though T is a template parameter of the class template. This happens because the template parameter T in the out-of-line definition of the constructor doesn't have the flag TEMPLATE_TYPE_PARM_FOR_CLASS set, and during duplicate_decls the the redeclaration (which is in terms of this unflagged T) prevails. To fix this, we could perhaps be more consistent about setting the flag, but it appears we don't really need this flag to make the determination. Since the template parameters of an synthesized guide consist of the template parameters of the class template followed by those of the constructor (if any), it should suffice to look at the index of the template parameter to determine whether it comes from the class template or the constructor (template). This patch replaces the TEMPLATE_TYPE_PARM_FOR_CLASS flag with this approach. PR c++/88252 gcc/cp/ChangeLog: * cp-tree.h (TEMPLATE_TYPE_PARM_FOR_CLASS): Remove. * pt.c (push_template_decl): Remove TEMPLATE_TYPE_PARM_FOR_CLASS handling. (redeclare_class_template): Likewise. (forwarding_reference_p): Define. (maybe_adjust_types_for_deduction): Use it instead. Add 'tparms' parameter. (unify_one_argument): Pass tparms to maybe_adjust_types_for_deduction. (try_one_overload): Likewise. (unify): Likewise. (rewrite_template_parm): Remove TEMPLATE_TYPE_PARM_FOR_CLASS handling. gcc/testsuite/ChangeLog: * g++.dg/cpp1z/class-deduction96.C: New test.
Fixed for GCC 12